Flat panel displays are widely used in a variety of applications, including computer displays. In addition to liquid crystal and plasma displays, one type of device well suited for such applications is a field emission display. Field emission displays typically include a generally planar substrate having an array of electron emitters. In many cases, the emitters are conical projections integral to the substrate.
FIG. 1 is a simplified side cross-sectional view of a portion of a field emission display 110 including a faceplate 120 and a baseplate 121 in accordance with the prior art. FIG. 1 is not drawn to scale. The faceplate 120 includes a transparent viewing screen 122, a transparent conductive layer 124 and a cathodoluminescent layer 126. The transparent viewing screen 122 supports the layers 124 and 126, acts as viewing surface and as a wall for a hermetically sealed package formed between the viewing screen 122 and the baseplate 121. The viewing screen 122 may be formed from glass or other transparent material. The transparent conductive layer 124 may be formed, for example, from indium tin oxide. The cathodoluminescent layer 126 may be segmented into localized portions. In a conventional monochrome display 110, each localized portion of the cathodoluminescent layer 126 forms one pixel of the monochrome display 110. Also, in a conventional color display 110, each localized portion of the cathodoluminescent layer 126 forms a green, red or blue sub-pixel of the color display 110. Materials useful as cathodoluminescent materials in the cathodoluminescent layer 126 include Y.sub.2 O.sub.3 :Eu (red, phosphor P-56), Y.sub.3 (Al,Ga).sub.5 O.sub.12 :Tb (green, phosphor P-53) and Y.sub.2 (SiO.sub.5):Ce (blue, phosphor P-47) available from Osram Sylvania of Towanda, Pa. or from Nichia of Japan.
The baseplate 121 includes emitters 130 formed on a planar surface of a substrate 132 that is preferably a semiconductor material such as silicon. The substrate 132 is coated with a dielectric layer 134. In one embodiment, this is effected by deposition of silicon dioxide via a conventional TEOS process. The dielectric layer 134 is formed to have a thickness that is approximately equal to or just less than a height of the emitters 130. This thickness is on the order of 0.4 microns, although greater or lesser thicknesses may be employed. A conductive extraction grid 138 is formed on the dielectric layer 134. The extraction grid 138 may be formed, for example, as a thin layer of polysilicon. An opening 140 is created in the extraction grid 138 having a radius that is also approximately the separation of the extraction grid 138 from the tip of the emitter 130. The radius of the opening 140 may be about 0.4 microns, although larger or smaller openings 140 may also be employed.
In operation, the extraction grid 138 is biased to a voltage on the order of 100 volts, although higher or lower voltages may be used, while the substrate 132 is maintained at a voltage of about zero volts. Signals coupled to the emitters 130 allow electrons to flow to the emitter 130. Intense electrical fields between the emitter 130 and the extraction grid 138 cause emission of electrons from the emitter 130.
A larger positive voltage, ranging up to as much as 5,000 volts or more but usually 2,500 volts or less, is applied to the faceplate 120 via the transparent conductive layer 124. The electrons emitted from the emitter 130 are accelerated to the faceplate 120 by this voltage and strike the cathodoluminescent layer 126. This causes light emission in selected areas, i.e., those areas opposite the emitters 130, and forms luminous images such as text, pictures, and the like.
The brightness of the light produced in response to the emitted electrons depends, in part, upon the number of electrons striking the cathodoluminescent layer 126 in a given interval. Field emission microscopy of the emitters 130 reveal that electrons are emitted from only a few atomic sites at the tip of the emitters. The emitting area is very small, generally from 1-5 nm in diameter. Uniformity in the shape, height, and placement of the emitters 130 is an important factor in the quality of the field emission display 110. These parameters affect differences in the number of electrons striking areas of the cathodoluminescent layer 126 that may be perceived by the viewer as bright and dark areas, or as other defects.
For instance, if an emitter 130 is shorter than other emitters, electrons emitted from the tip of the taller emitter may have a tendency to spread out more as they are directed to the cathodoluminescent layer 126. This could cause electrons to bleedover to areas of the cathodoluminescent layer 126 other than those intended, creating a picture defect. Similarly, emitters 130 that are longer than the others may have a tendency to not spread out as much as desired. Mis-located emitters 130 may tend to create a surplus of electrons in one area and a deficiency of electrons in others, also making a deficient picture.
Arrays of emitters 130 can be formed by chemical mechanical polishing steps such as those taught in U.S. Pat. No. 5,372,973, assigned to Micron Technology, Inc. and incorporated herein by reference. These arrays of emitters 130 can also be formed by typical semiconductor fabrication processes such as wet or dry etching of the silicon substrate 132. One example of forming emitters 130 by semiconductor fabrication steps is seen in U.S. Pat, No. 5,766,829 assigned to Micron Technology, Inc. and incorporated herein by reference. In the '829 patent, printed features for defining the size and location of emitter sites are made using phase shift lithography. As seen in FIG. 2 of the '829 patent, by using this method, the phase of exposure energy such as visible light or x-rays is controlled through a reticle in two orientations so that exposed and non-exposed regions or "islands" are produced on a photoresist by destructive or constructive interference. The islands are hardened and then used as etching masks. Isotropic or anisotropic etching is performed on the exposed substrate, while leaving the areas under the islands intact. Etching continues until the areas of the substrate under the islands form points; then the islands are removed. These points become the emitters of the flat panel display.
A problem in using phase shift lithography is that it is difficult to control the photoresist onto which the exposure energy is directed, causing the islands formed on the baseplate to be non-uniform. Later processing with nonuniform islands cause nonuniform emitters to be formed, and ultimately creates a substandard field emission display.
Other semiconductor fabrication technologies have been used to make arrays of emitters 130. For instance, a negative photoresistive material layered on the substrate has been used. Using a negative photoresist to make an array of emitters 130 requires exposing the photoresist only where the islands are to be formed. The exposing energy directed to the negative photoresist hardens the exposed areas and later developing removes the nonexposed areas. This creates an array of islands of exposed photoresist for later processing into an array of emitters 130. However, using a negative photoresist is disfavored for many reasons. It is extremely temperature sensitive, so that normal variations in processing temperatures create nonuniform islands. Some negative photoresist has a tendency to swell during developing, thus preventing its use in very small pattern making. It also has a limited depth of focus. Additionally, developing some negative photoresist requires organic solvents that are flammable as well as difficult and expensive to safely dispose.
A positive photoresistive material can be layered on the substrate, patterned then exposed, but this process also has difficulties. When using a positive photoresist, areas that receive the exposing energy are removed and areas that are shielded from the exposing energy remain after developing. In order to form an array of small islands, most of the positive photoresist is exposed, e.g., over 95%. Trying to create uniform islands of non-exposed positive photoresist is difficult with so much exposing energy applied to the positive photoresist. For instance, if the exposing energy is visible light, excess light tends to undercut the mask, thereby exposing the positive photoresist meant to be covered. In addition, the exposing light is reflected, refracted, and scattered around the photoresist. The same effects occur with x-ray or other exposing energies during exposing times. Unfortunately, these effects are nonuniform which causes the islands of positive photoresist to be nonuniform as well. As described above, it is impossible to create uniform emitters 130 from nonuniform islands.
Thus, it would be desirable to devise a method for creating uniform emitters using fabrication steps that are currently existing.